dependabot: explicitly exclude rust-vmm deps from firecracker group

Apparently, if a rust-vmm dep (currently vmm-sys-util) is included in a
PR for the firecracker group _once_, it will indefinitely be stuck in
there. So explicitly exclude these deps from that group, in hopes of
getting dependabot to understand that it should split the current PR
into two.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.github/dependabot.yml

@@ -22,3 +22,13 @@ updates:
       firecracker:
         patterns:
           - "*"
+        exclude-patterns:
+          - "vmm-sys-util"
+          - "kvm-bindings"
+          - "kvm-ioctls"
+          - "vm-memory"
+          - "vhost"
+          - "linux-loader"
+          - "vm-allocator"
+          - "event-manager"
+          - "vm-superio"